Handbook

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Using DSCP values to provide QoS
The six most significant bits in the TOS byte of the IP header are defined as DiffServ Code Points (DSCP).
Packets are marked with a certain value depending on the type of treatment the packet must receive in
the network device. DSCP is a measure of the Quality of Service (QoS) level of the packet.
Differentiated Services concepts
To differentiate between traffic flows, packets can be classified by their DSCP value. The Differentiated
Services (DS) field in the IP header is an octet, and the first six bits, called the DS Code Point (DSCP), can
provide QoS functions. Each packet carries its own QoS state in the DSCP. There are 64 possible DSCP
values (0-63).
Figure 12 Layer 3 IPv4 packet
The switch can perform the following actions to the DSCP:
Re-mark the DSCP value to a new value
Map the DSCP value to an 802.1p priority
Per Hop Behavior
The DSCP value determines the Per Hop Behavior (PHB) of each packet. The PHB is the forwarding
treatment given to packets at each hop. QoS policies are built by applying a set of rules to packets,
based on the DSCP value, as they hop through the network.
The switch default settings are based on the following standard PHBs, as defined in the IEEE standards:
Expedited Forwarding (EF) : This PHB has the highest egress priority and lowest drop precedence
level. EF traffic is forwarded ahead of all other traffic. EF PHB is described in RFC 2598.
Assured Forwarding (AF) : This PHB contains 4 service levels, each with a different drop precedence,
as shown below. Routers use drop precedence to determine which packets to discard last when the
network becomes congested. AF PHB is described in RFC 2597.
Table 17 Assured forwarding drop-down precedence
Drop Precedence
Class 1
Class 2
Class 3
Class 4
Low
AF11 (DSCP 10)
AF21 (DSCP 18)
AF31 (DSCP 26)
AF41 (DSCP 34)
Medium
AF12 (DSCP 12)
AF22 (DSCP 20)
AF32 (DSCP 28)
AF42 (DSCP 36)
High
AF13 (DSCP 14)
AF23 (DSCP 22)
AF33 (DSCP 30)
AF43 (DSCP 38)